1. Field of Invention
The present invention relates to tennis racket stringing machines.
2. The Prior Art
Many machines have been devised for stringing and restringing game rackets, such as those used for tennis, badminton, squash and the like.
After 1969, a process that had previously been done by guess, intuition or the displacement of fixed weights (Serrano, U.S. Pat. No. 2,188,250) became more efficient and precise by using the compression of a spring with its inherent linearity (Held, U.S. Pat. No. 3,441,275) as a comparator. Here the stringing machine FIG. 15 holds the racket in a cradle in a position parallel to the ground 130. The person stringing a racket threads the string through a hole in the racket frame, attaching one end to the racket and the other to an external self-tightening vise 131 (snatch vise). The vise is part of a hand cranked tensioning assembly 132 (tension head) that automatically brakes when the tension on the string equals the tension preset on a helical bias spring. The tension head runs on a track 133 that draws the string away from the racket while tensioning. This is the so-called Pull and Brake method.
Modified, Held's device is still used universally although its accuracy is often called into question, its resolution is limited and it needs frequent calibration. In substantially similar forms this machine is manufactured by Ektelon, Gamma, Alpha, Czech Sports, Eagnas, Toalson, Gossen, Kennex, Winn and others.
From 1975, machines surfaced that used electric motors to replace the hand crank that compresses the bias spring (Kaminstein, U.S. Pat. No. 3,918,713), (Tsuchida, U.S. Pat. No. 4,620,705) and (Muselet et al., U.S. Pat. No. 4,376,535). Some machines used hydraulics or pneumatic systems as the power source (Morrone, U.S. Pat. No. 4,417,729).
When wooden rackets became obsolete, rackets of aluminum, graphite, boron, ceramic, Kevlar, etc. made their appearance along with hundreds of kinds of new strings made of different plastics and multi-layered filaments. Improvements to the equipment required an improvement in the accuracy of the tools needed for their stringing and thus electronic machines.
Babolat of France (U.S. Pat. No. 5,026,055) and Poreex of Taiwan (U.S. Pat. No. 5,090,697) manufacture essentially duplicate electronic machines sold under their own name and brand labeled for others. In their device the snatch vise is driven by a spring-loaded chain drive.
Not unlike earlier machines the chain drive compresses a helical spring. Running parallel to this bias spring is a linear potentiometer. The electronics read the linear potentiometer as it measures the spring compression and indirectly the tension on the string through the intermediary of the chain/spring/potentiometer assembly.
All electronic machines are "Constant Pull" machines and continue to apply tension even after the dialed-in tension is reached because strings lose some tension seconds after their initial pull. This Constant Pull feature is often the cause of undesirable results. Knowledgeable players ask their stringer which machine will be used to string their racket, mechanical (Pull and Brake) or electronic (Constant Pull). The results can be substantially different. Electronic machines will invariably produce a racket that is 5-10 percent tighter (where it appears as if more tension has been applied to the strings) than a Pull and Brake machine. Professional players claim they can feel the difference in small fractions of a pound.